I'm having a problem determining what makes a print "bright" and "dark". It seems like an easy concept but it isn't for me. I need to understand this accurately.

I can "instantly" tell by looking, if the image looks "bright" or "dark". (sorry about all the quoting...) But! What makes these prints bright or dark? If I take a given negative and print it with less exposure, it will be a THIN print or an under-exposed print. If I take a given negative and print it with more exposure, it will be a DENSE print or an over-exposed print. Either way, they look WRONG. There is a narrow range where I can adjust the exposure and make the print look "correct."

I looked at this from average density stand point. That doesn't seem to be the cause either.

Is it a local contrast around the subject?
Is it a global contrast, as in use of different contrast filter?
Is it the subject being lighter than the rest?

Can someone explain the mechanics behind how all of this work?